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You can think of NASA's Discovery program as a sort of outer-space American Idol: every few years the agency invites scientists to propose unmanned planetary missions. The projects have to address some sort of fundamental science question, and (this is the tough part) they have to be relatively cheap to pull off  say, half a billion dollars or so. Then the proposals go through a grueling competition before judges who aren't as nasty as Simon Cowell but who are every bit as tough. The one left standing at the end gets the equivalent of a recording contract: NASA supplies the funding and the launch vehicle, and away the winner goes  to orbit Mercury, as the Messenger spacecraft is doing right now; or to rendezvous with a couple of asteroids, as the Dawn mission will start doing this July; or to smash into a comet on purpose, a feat achieved by Deep Impact in 2005, a mission not to be confused with the movie of the same name.

Now it's time for the next contenders. NASA has just announced that the first round of the latest Discovery competition is over, with three entries out of 28 moving on to the finals. They are, in increasing distance from Earth: the Geophysical Monitoring Station (GEMS) lander, which would use seismometers to study the interior of Mars; the Comet Hopper, which would do just that, leaping from place to place across the surface of Comet 46P/Wirtanen to see how different parts of the tumbling body react to heating by the sun; and the Titan Mare Explorer (TiME), which would plop into a sea of liquid hydrocarbons on Saturn's moon Titan  the first oceangoing vessel ever to set sail on another world.
(See the 40th anniversary of the moon landing.)

If you had to come up with a theme that ties all three missions together, it would be "origins." The Titan explorer, for example, will be studying a place that  in a crude way, at least  resembles the early planet Earth at a time when life arose here. Titan, with a thick atmosphere and a bizarro-world form of weather featuring toxic winds and hydrocarbon rain, is home to a mix of complex chemistry, complete with organic molecules. The oceans provide a medium in which the molecules can move around and interact with each other. It's even conceivable, though clearly a long shot, that some form of microscopic life already exists on this frigid moon.

The Mars lander, by contrast, would visit a place where the seas  plain water in this case  vanished long ago. But the mission of GEMS goes far deeper than that. By analyzing Marsquakes on the Red Planet, GEMS will try to get a handle on what the interior of Mars is like. Scientists don't currently know whether the planet's core is liquid, like Earth's, or solid, or some mushy consistency in between. It all depends on how efficiently Mars has cooled since it formed 4.5 billion years ago, and that depends in turn on the planet's internal structure.
(See pictures of the Martian landscape.)

"That's the mission," says Bruce Banerdt, of NASA's Jet Propulsion Laboratory, the lead scientist for GEMS. "We want to understand how Mars was built." Along with sensitive seismographic equipment, GEMS will drill down about 20 ft. (6 m) with a thermometer-equipped probe, trying to figure out how quickly the temperature rises with depth. "That will let us extrapolate all the way down to the center," Banerdt says, "which will tell us how fast Mars is cooling."

As for the Comet Hopper, if it gets the green light, it will be visiting an object that's like a time capsule from the earliest days of the solar system. The planets and the major moons have all been squeezed and heated by gravity and pummeled by giant impacts, erasing much of their history. Comets, by contrast, are relatively pristine  at least, below their outer crust. As 46P/Wirtanen is warmed by the sun, some of that subsurface material heats up and spews out. By being there to sample it  and especially by sampling it at different points on the surface  the Hopper will be taking a whiff of the solar system's original material.
(See pictures of Titan.)

Now, says Paul Hertz, chief scientist at NASA's Science Directorate in Washington, D.C., comes the hard part. "All three missions are absolutely worth doing, all of them have very good plans," he says. Or at least, they have good concepts. "The teams have only done a little bit of work so far," he says. (Many of NASA's million or so Twitter followers have already begun tweeting their own votes, but these, unsurprisingly, won't count.)

The next round of proposals, which NASA is funding to the tune of $3 million each, and which are due in nine months or so, have to be a lot more detailed. Each will have to pass muster with two different panels of experts: a group of scientists who will take a hard look at the potential payoffs being touted by each team, and a group of engineers and project managers who will look with professionally skeptical eyes at the proposed budgets. "We'll be paying a lot of attention to bringing these in on budget," says Hertz. "In the end, it will probably come down to how believable their plans are."

Whoever wins gets the grand prize: NASA's blessing and, more important, $425 million (in 2010 dollars) to complete the mission. But each of the finalists already has Hertz's respect. "If you give the science community a challenge," he says, "they're really creative and clever about doing great science."